build: add the ability to create a symlink for dsymutil

Add a `LLVM_INSTALL_CCTOOLS_SYMLINKS` to mirror
`LLVM_INSTALL_BINUTILS_SYMLINKS`.  For now, this allows us to create
symlinks for `dsymutil` to `llvm-dsymutil`.  This option is off by
default, but the user can enable it.

llvm-svn: 326381